fix(api): align public SamplingParam ltx2 defaults with distilled

Internal FastVideo declares two LTX-2 SamplingParam variants:
LTX2SamplingParam (mod=3.0/rescale=0.7/stg=1.0 — full LTX-2) and
LTX2DistilledSamplingParam (mod=1.0/rescale=0.0/stg=0.0). Dreamverse
production runs against LTX2-Distilled-Diffusers, so the distilled
defaults are what the runtime should land on.

The public package only has the single SamplingParam class, and its
class-level defaults for these knobs were the *full* LTX-2 values.
Result: when Dreamverse (or any other consumer) called
``SamplingParam.from_pretrained("FastVideo/LTX2-Distilled-Diffusers")``,
the LTX2_DISTILLED preset's defaults dict didn't override these
fields (it didn't list them) so they fell through to the class
defaults — silently picking up *full*-model CFG / modality / rescale /
STG instead of the distilled ones. Streamed video output diverged
from the internal-ui reference.

Switch the class defaults to the distilled values (1.0 / 0.0 / 0.0).
The LTX2_BASE preset already overrides them explicitly to 3.0/0.7/1.0
in its ``defaults`` dict, so users selecting the full-LTX-2 preset
keep the full-model behavior.

The numerical alignment harness still reproduces the same 42.72 dB
PSNR / 52.87% exact pixel match because that test pinned the values
explicitly via legacy kwargs; this fix moves the *default* path on
the public side onto the same numerical track.

Co-authored-by: Junda (David) Su <90978028+Davids048@users.noreply.github.com>
Co-authored-by: Matthew Noto <99706358+RandNMR73@users.noreply.github.com>
Co-authored-by: XOR-op <17672363+XOR-op@users.noreply.github.com>
Co-authored-by: Zhang Peiyuan <42993249+jzhang38@users.noreply.github.com>

fastvideo/api/sampling_param.py

@@ -98,17 +98,22 @@ class SamplingParam:
     camera_rotation: str | None = None
 
     # LTX-2 multi-modal CFG and STG.
-    # cfg_scale defaults are 1.0 (CFG off) so ``ForwardBatch.__post_init__``
-    # doesn't force ``do_classifier_free_guidance`` on non-LTX-2 models that
-    # never override these fields. LTX-2 presets that need text-CFG on set
-    # them in their ``defaults`` dict (e.g. ``ltx2_base``).
+    # Class-level defaults match the *distilled* LTX-2 schedule
+    # (mirrors ``FastVideo-internal/.../LTX2DistilledSamplingParam``):
+    # the distilled model expects neutral guidance scales — modality 1,
+    # rescale 0, STG 0 — and explicit-CFG callers (full LTX-2) opt back
+    # in by selecting the ``LTX2_BASE`` preset, which overrides these
+    # to mod=3.0 / rescale=0.7 / stg=1.0 in its ``defaults`` dict.
+    # cfg_scale defaults stay at 1.0 (CFG off) so
+    # ``ForwardBatch.__post_init__`` doesn't force CFG on non-LTX-2
+    # models that never override these fields.
     ltx2_cfg_scale_video: float = 1.0
     ltx2_cfg_scale_audio: float = 1.0
-    ltx2_modality_scale_video: float = 3.0
-    ltx2_modality_scale_audio: float = 3.0
-    ltx2_rescale_scale: float = 0.7
-    ltx2_stg_scale_video: float = 1.0
-    ltx2_stg_scale_audio: float = 1.0
+    ltx2_modality_scale_video: float = 1.0
+    ltx2_modality_scale_audio: float = 1.0
+    ltx2_rescale_scale: float = 0.0
+    ltx2_stg_scale_video: float = 0.0
+    ltx2_stg_scale_audio: float = 0.0
     ltx2_stg_blocks_video: list[int] = field(default_factory=lambda: [29])
     ltx2_stg_blocks_audio: list[int] = field(default_factory=lambda: [29])